Field of the Invention
The present invention relates to a quantum cascade detector using intersubband light absorption in a quantum well structure.
Related Background Art
Since there are absorption lines corresponding to fundamental vibrations of a number of gas molecules in a mid-infrared region (for example, a region of a wavelength of 4 to 10 μm), mid-infrared light has been applied in the infrared absorption spectroscopy. In particular, when a laser light source is used as a mid-infrared light source, only an absorption line of a particular gas type is observed by using its narrow band emission line width, whereby high sensitivity absorption spectroscopy can be achieved without being disturbed by components of other gas types.
As a photodetector in a mid-infrared wavelength region, an MCT (HgCdTe) detector, an InSb detector, a quantum well infrared photodetector (QWIP), and the like have been known, and for a spectroscopic experiment for measuring weak mid-infrared light, a liquid nitrogen cooling type MCT detector or InSb detector has been mainly used. However, each of the MCT detector and the InSb detector requires application of an external voltage for driving, and noise (dark current) is generated due to the voltage application. Therefore, in these detectors, suppression of the noise has been a problem to improve S/N characteristics and sensitivity of spectroscopic measurement. In addition, the MCT detector has a problem such that it contains a toxic material, and is difficult to be used generally.
Meanwhile, in recent years, as a photodetector in the mid-infrared region, a quantum cascade detector (QCD) using a cascade structure has been proposed. The quantum cascade detector is capable of taking out photoelectric current without application of the external voltage, by controlling intersubband transitions with a design of a quantum well structure in an active layer. For this reason, the quantum cascade detector does not generate a noise component caused by the external voltage, and is expected as an extremely low-noise photodetector. In addition, the quantum cascade detector can be configured by general semiconductor materials, so that a problem of a toxic material such as in the MCT detector does not occur (for example, see Non Patent Documents 1 to 3).    Non Patent Document 1: F. R. Giorgetta et al., “Quantum Cascade Detectors”, IEEE Journal of Quantum Electronics Vol. 45 No. 8 (2009) pp. 1039-1052    Non Patent Document 2: A. Harrer et al., “Plasmonic lens enhanced mid-infrared quantum cascade detector”, Appl. Phys. Lett. Vol. 105 (2014) pp. 171112-1-171112-4    Non Patent Document 3: B. Schwarz et al., “Monolithically integrated mid-infrared lab-on-a-chip using plasmonics and quantum cascade structures”, Nat. Commun. Vol. 5 Art. 4085 (2014) pp. 1-7